The estimation of pregnancy dates is important for the mother, who wants to know when to expect the birth of her baby, and for her health care providers, so they may choose the times at which to perform various screening tests and assessments,such as serum screening, assessment of maturity, and induction of labor for postdate pregnancies.
The 3 basic methods used to help estimate gestational age (GA) are menstrual history, clinical examination, and ultrasonography.
Gestational age (GA) has traditionally been estimated from the date of the last menstrual period (LMP).
This estimation assumes that conception occurs on day 14 of the cycle.
The GA of pregnancies resulting from in vitro fertilization can be precisely calculated from the time of embryo replacement; however, conception may be delayed for a few days in pregnancies resulting from intrauterine insemination.Therefore, giving a range for the likely date of birth (eg, EDD -2 weeks to EDD 1 week) is more useful.Infants born before 37 completed weeks' gestation are deemed preterm, whereas those born after 42 weeks' are considered post-term.Knowing the date of the first positive pregnancy test result allows the calculation of a minimum GA. For example, if the test was performed 4 weeks ago and the test is known to return positive results as early as 1 week after conception, then the minimum conceptional age (CA) would be 5 weeks (GA, 5 2 = 7 wk of amenorrhea).This information can be useful in clinical practice if the test finding has been documented by a health care professional.